<!-- Licensed under a BSD license. See license.html for license -->
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0, user-scalable=yes">
<title>WebGL - 2D Rectangles</title>
<link type="text/css" href="resources/webgl-tutorials.css" rel="stylesheet" />
</head>
<body>
<canvas id="canvas"></canvas>
</body>
<!-- vertex shader -->
<script id="vertex-shader-2d" type="x-shader/x-vertex">
attribute vec2 a_position;

uniform vec2 u_resolution;

void main() {
   // convert the rectangle from pixels to 0.0 to 1.0
   vec2 zeroToOne = a_position / u_resolution;

   // convert from 0->1 to 0->2
   vec2 zeroToTwo = zeroToOne * 2.0;

   // convert from 0->2 to -1->+1 (clipspace)
   vec2 clipSpace = zeroToTwo - 1.0;

   gl_Position = vec4(clipSpace * vec2(1, -1), 0, 1);
}
</script>
<!-- fragment shader -->
<script id="fragment-shader-2d" type="x-shader/x-fragment">
precision mediump float;

uniform vec4 u_color;

void main() {
   gl_FragColor = u_color;
}
</script>
<!--
for most samples webgl-utils only provides shader compiling/linking and
canvas resizing because why clutter the examples with code that's the same in every sample.
See https://webglfundamentals.org/webgl/lessons/webgl-boilerplate.html
and https://webglfundamentals.org/webgl/lessons/webgl-resizing-the-canvas.html
for webgl-utils, m3, m4, and webgl-lessons-ui.
-->
<script src="resources/webgl-utils.js"></script>
<script>
"use strict";

function main() {
  // Get A WebGL context
  /** @type {HTMLCanvasElement} */
  var canvas = document.querySelector("#canvas");
  var gl = canvas.getContext("webgl");
  if (!gl) {
    return;
  }

  // setup GLSL program
  var program = webglUtils.createProgramFromScripts(gl, ["vertex-shader-2d", "fragment-shader-2d"]);

  // look up where the vertex data needs to go.
  var positionAttributeLocation = gl.getAttribLocation(program, "a_position");

  // look up uniform locations
  var resolutionUniformLocation = gl.getUniformLocation(program, "u_resolution");
  var colorUniformLocation = gl.getUniformLocation(program, "u_color");

  // Create a buffer to put three 2d clip space points in
  var positionBuffer = gl.createBuffer();

  // Bind it to ARRAY_BUFFER (think of it as ARRAY_BUFFER = positionBuffer)
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  webglUtils.resizeCanvasToDisplaySize(gl.canvas);

  // Tell WebGL how to convert from clip space to pixels
  gl.viewport(0, 0, gl.canvas.width, gl.canvas.height);

  // Clear the canvas
  gl.clearColor(0, 0, 0, 0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Tell it to use our program (pair of shaders)
  gl.useProgram(program);

  // Bind the position buffer.
  gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);

  // create the buffer
  const indexBuffer = gl.createBuffer();

  // make this buffer the current 'ELEMENT_ARRAY_BUFFER'
  gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);

  // Fill the current element array buffer with data
  const indices = [
    0, 1, 2,   // first triangle
    2, 1, 3,   // second triangle
  ];
  gl.bufferData(
      gl.ELEMENT_ARRAY_BUFFER,
      new Uint16Array(indices),
      gl.STATIC_DRAW
  );

  // code above this line is initialization code
  // --------------------------------
  // code below this line is rendering code

  // Turn on the attribute
  gl.enableVertexAttribArray(positionAttributeLocation);

  // Tell the attribute how to get data out of positionBuffer (ARRAY_BUFFER)
  var size = 2;          // 2 components per iteration
  var type = gl.FLOAT;   // the data is 32bit floats
  var normalize = false; // don't normalize the data
  var stride = 0;        // 0 = move forward size * sizeof(type) each iteration to get the next position
  var offset = 0;        // start at the beginning of the buffer
  gl.vertexAttribPointer(
      positionAttributeLocation, size, type, normalize, stride, offset);

  // bind the buffer containing the indices
  gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);

  // set the resolution
  gl.uniform2f(resolutionUniformLocation, gl.canvas.width, gl.canvas.height);

  // draw 50 random rectangles in random colors
  for (var ii = 0; ii < 50; ++ii) {
    // Setup a random rectangle
    // This will write to positionBuffer because
    // its the last thing we bound on the ARRAY_BUFFER
    // bind point
    setRectangle(
        gl, randomInt(300), randomInt(300), randomInt(300), randomInt(300));

    // Set a random color.
    gl.uniform4f(colorUniformLocation, Math.random(), Math.random(), Math.random(), 1);

    // Draw the rectangle.
    var primitiveType = gl.TRIANGLES;
    var offset = 0;
    var count = 6;
    var indexType = gl.UNSIGNED_SHORT;
    gl.drawElements(primitiveType, count, indexType, offset);
  }
}

// Returns a random integer from 0 to range - 1.
function randomInt(range) {
  return Math.floor(Math.random() * range);
}

// Fill the buffer with the values that define a rectangle.
function setRectangle(gl, x, y, width, height) {
  var x1 = x;
  var x2 = x + width;
  var y1 = y;
  var y2 = y + height;
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
     x1, y1,
     x2, y1,
     x1, y2,
     x2, y2,
  ]), gl.STATIC_DRAW);
}

main();
</script>
</html>


